The present invention generally relates to vehicle stability control, and more particularly relates to a method and a device for detecting the risk of rollover in vehicles which are equipped with a driving stability control system.
It has been known for long that in vehicles having a high center of gravity and/or a small tread width, for example trucks, track trailer units, buses, small buses, and off-road vehicles, there is a rollover risk during cornering with a major roll motion. Various models for rollover accidents are described on pages 309 to 333, chapter 9, of the book xe2x80x9cFundamentals of Vehicle Dynamicsxe2x80x9d, by T. D. Gillespie, Society of Automotive Engineers, Inc., Warrendale 1992, which is referred to in full extent in the present context. Starting with a quasi-stationary model for a rigid vehicle by way of a quasi-stationary model for a sprung vehicle up to dynamic models in consideration of inherent roll frequencies, conditions for existing rollover hazards are indicated.
However, it has been found more recently that lateral motions of passenger vehicles may also build up until the vehicle rolls over. Such a risk of rollover is considerably increased by improper loading, e.g. extremely on one side or on the vehicle roof, because the center of gravity of the mass of the vehicle is shifted upwards or to one side. In addition, it is that more frequently vehicles are registered in recent times which are designed as passenger cars with a relatively high center of gravity, for example, the new vehicle class of the so-called xe2x80x9cvansxe2x80x9d.
To explain the physical driving conditions underlying rollover, FIG. 2a shows a schematic rear view of a vehicle 210 standing on a roadway 200. Reference numerals 103 and 104 designate the wheels of the rear axle. It is assumed that the vehicle is in a left turn, thus would move to the left in a projection to the drawing plane. The circular travel of the vehicle produces a centrifugal force Z=mwxc3x97xcfx892xc3x97r=mxc3x97v2:r, wherein m is the vehicle mass, xcfx89 the angular velocity during circular travel, v the vehicle speed and r the radius of the circular travel. The acting centrifugal force Z which can be represented as a product aqxc3x97m, with aq being the transverse acceleration, can be assumed to act on the center of gravity S of the vehicle. The center of gravity S is disposed roughly centrically between the wheels and at a height h above the roadway. Acting on the center of gravity S is also the weight force G=mxc3x97g, wherein g is the gravitational acceleration. As long as the vehicle drives on the desired circle (aq=v2:r applies then), i.e., as long as the cornering forces F on the four wheels (roughly corresponding to F=xcexcxc3x97G, wherein xcexc is the coefficient of friction between tires and roadway) equals the centrifugal force Z, the above-mentioned centrifugal forces will develop according to the above equation. It may happen then that the vehicle will roll over the outside wheel due to an unfavorable torque distribution. This will principally happen when Gxc3x97b:2 less than Zxc3x97h applies, wherein h is the height of the center of gravity S above the roadway 200 and b:2 is roughly half the tread width of the vehicle. The above inequation is a first approximation of the torque equilibrium about the point P. When the outwards turning torque Zxc3x97h is greater than the inwards turning torque Gxc3x97b:2, the vehicle will roll over outwards. This risk is encountered especially with vehicles having a small tread width (b:2) and a comparatively great height and, thus, a high center of gravity (high value of a), e.g. caused by a roof load 220 on the vehicle 210.
To effectively avoid a like operating condition, it is necessary to
detect a critical situation, especially a driving condition with a critical transverse acceleration, and
take appropriate countermeasures following the detection.
In conventional driving dynamics control systems, for example, the ESP system (Electronic Stability Program) by the applicant, driving dynamics parameters, among others the transverse acceleration, the time variation of the transverse acceleration, or the tire slip angle, are provided as driving dynamics parameters which are indicative of the tendency of vehicle rollover about the longitudinal axis of the vehicle. A corresponding method for the operation of a vehicle with brake interventions that stabilize driving is e.g. described in German published patent application DE-A 196 32 943 xe2x80x9cMethod for the operation of an automotive vehicle with brake interventions that stabilize drivingxe2x80x9dDaimler-Benz Aktiengesellschaft, wherein the transverse acceleration is taken into consideration as the only driving-dynamics parameter indicative of the vehicle""s tendency to rollover about the longitudinal vehicle axis. An associated predefinable rollover prevention threshold value is provided for the transverse acceleration.
During cornering, the vehicle is kept to track by the transverse forces which act at the tire tread surfaces on the roadway. The largest part of these transverse forces is produced by the curve-outward wheels or tires. When the transverse acceleration which occurs during cornering exceeds the rollover prevention threshold value, the curve-outward wheels will adopt a condition of high brake slip caused by activation of a corresponding brake intervention, with the result that the transverse force that can be transmitted by the tires is considerably reduced. Consequently, the curve-outward wheels can no longer withstand the transverse acceleration acting upon them (which will possibly increase the sideslip angle and turn the vehicle front end or the vehicle rear end slightly in the direction of the transverse acceleration torque). However, simultaneously, the rollover torque is reduced and rollover of the vehicle about its longitudinal axis prevented. In addition to this, the above-mentioned publication discloses an embodiment wherein the time variation of the transverse acceleration is taken into account as an indicative driving-dynamics parameter.
In published patent application DE-A 197 46 889 xe2x80x9cVehicle Motions Control Systemxe2x80x9d, by Aisin Seiki K.K. et al, a system for increasing the lateral stability of an automotive vehicle during cornering is described wherein there is provision of a rollover detection unit for detecting a rollover motion of a normal axle of the vehicle with respect to the vehicle""s vertical axis and a cornering determining unit for determining a cornering condition of the vehicle. To calculate the vehicle rollover motion or the vehicle rollover, either the difference in height between the right and the left vehicle side or the transverse acceleration of the vehicle is detected to establish the roll angle between the horizontal vehicle line and the horizontal roadway line. A linearity between the transverse acceleration aq and the vehicle rollover designated by a roll angle gamma is made the basis. When the inclination detection device detects a rollover risk, a countersteering yaw torque is produced by slowing down the curve-outward front wheel.
As has been described hereinabove though, the allowed transverse acceleration and the allowed roll angle depend on the center of gravity of the vehicle, especially the height of the vehicle center of gravity.
The known methods and systems for detecting the vehicle inclination or the roll angle, on the one hand, include the shortcoming that they require additional sensor means, for example, in the event of an inclination detection device with quantities that determine the difference in level between the right and the left vehicle sides, or that they depend on current vehicle characteristics such as the load condition or the center of gravity of the entire vehicle and, consequently, are subject to the requirement of constantly updating the basic vehicle data.
In view of the above, an object of the present invention is to provide a method and a device which overcome the above-mentioned disadvantages, i.e., which obviate the need for any additional sensor means and, in addition, are virtually independent of given vehicle characteristics or quantities.
To achieve this object, it is provided in the method of the present invention and the related device that the component of the transverse acceleration which generally acts in the horizontal plane of the vehicle is detected during cornering by means of the transverse acceleration sensor means, that a condition variable which is correlated to the centrifugal acceleration that acts on the center of gravity is determined, and that the roll angle of the vehicle is calculated from the difference between the detected component of the transverse acceleration and the determined centrifugal acceleration, the said difference being weighted with a factor.
Thus, the present invention is based on the concept of using axe2x80x94principally disadvantageous characteristicxe2x80x94characteristic of transverse acceleration sensors, i.e., the limitation of the measurement range in the predetermined horizontal plane of the vehicle, in such a manner that precise indications of the current rollover angle of the vehicle can be concluded from the deviation of this measured variable and the actual transverse acceleration which acts on the vehicle""s center of gravity (=centrifugal force). This deviation can be seen in particular in a measurable difference of the absolute values of these two transverse acceleration values.
The special advantage of using the rollover (roll) angle in sensing the rollover risk of a vehicle includes that this angle permits more precise statements about the imminent rollover risk than, for example, the transverse vehicle acceleration does. This is because the judgment of the rollover risk based on the roll angle requires no further model analyses other than the knowledge of the center of gravity of the vehicle. In contrast thereto, however, the generic methods known from the state of the art wherein the transverse acceleration as an output quantity is mostly made the basis, require further model analyses to be able to infer the rollover risk from the transverse acceleration data.
More particularly, the present invention avoids the necessity of technically complicated devices for determining the roll (rollover) angle and exclusively makes use of existing measured quantities and vehicle parameters. In addition, rollover sensing by way of the roll angle is independent of the majority of vehicle and roadway characteristics and, therefore, can favorably be achieved in a simple and low-cost manner. Variable vehicle parameters such as the current center of gravity which becomes a variable quantity due to the current load condition (maximum passenger and roof load) are not taken into consideration as values in rollover sensing.
In addition, the quantities obtained in the rollover sensing can be used directly as input data in an ESP system, that means, the present invention may be implemented in an especially favorable manner as another feature or function of the ESP.
Further advantages are involved because accident prevention is possible already before the curve-inward wheels actually lift off when rollover begins. In addition, dynamic driving situations can be taken into account with respect to the roll angle velocity.
In a favorable aspect of the present invention, it may further be provided that upon determining the condition variable correlated to the centrifugal acceleration, the difference of the wheel speeds on at least two axles is determined and a plausibility analysis is performed by comparison of the so obtained condition variables. Due the two independently found data, a check of the data produced can be effected to the end that vehicle conditions where e.g. only one single wheel loses the road contact, the resulting, apparently very great rotational speed differences and, hence, transverse accelerations can be interpreted as measurement errors and will in this case suitably prevent intervention of the ESP, for example.